The present invention relates generally to amplifiers, and in particular to a multi-band, configurable, variable gain, low noise amplifier (LNA) for radio frequency (RF) reception.
The LNA is an important component in wireless communication systems. Mobile terminals require a LNA to amplify very weak signals received at an antenna to a level sufficient for further processing, such as downconverting (mixing), demodulating, and decoding. Due to the mobile terminals' varying distance from a transmitter, shielding, fading, and other effects, RF signals are received at varying power levels. A variable gain LNA is desirable for properly amplifying the various received signals. For example, very weak signals require more amplification, and hence a higher gain, to reach an acceptable signal-to-noise ratio. However, a high-gain LNA will cause clipping of signals received with a higher power level, hence a lower gain is required.
Variable gain LNAs are known in the art. However, conventional variable gain LNA designs offer only very coarse gain adjustment, such as operating in either a low gain or high gain mode. Additionally, some conventional variable gain LNAs achieve the variation in gain by varying a resistance value connected in parallel with an inductive load. This introduces thermal noise into the amplified signal due to the physical resistive elements.
The application of weak RF signals also requires (as the name implies) low noise figures. Conventional fixed LNAs have different noise characteristics in different frequency bands, depending on the frequency for which the circuit is optimized. For multi-band RF receivers employing separate LNAs for different frequency bands, acceptable noise performance may require different designs for each LNA, optimizing the noise performance of each LNA for a particular frequency band. Multiple LNA designs in a single receiver increase development time and cost.
Another requirement of a LNA is linearity, to avoid interference. The linearity of a LNA is related to both over-drive voltage between the gate and source of a field effect transistor (FET), and FET drain current. Generally, increased over-drive voltage yields better linearity, at the price of higher power consumption. For long channel FET devices, the drain current is proportional to transistor width and the square of over-drive voltage. The gain of the LNA, Av, is proportional to the transconductance gm which is related to the square root of the product of drain current Id and transistor width W:
            A      v        ∝          g      m        =                    2        ⁢                  μ          n                ⁢                  C          ox                ⁢                  WI          d                    L      Where μn is mobility of electron, Coxis gate capacitance per unit area, and L is the channel length, respectively.
The gain can be adjusted by altering either the transistor width W or the drain current Id. However, conventional LNAs either cannot alter both parameters, or can only do so in very coarse steps, e.g., high gain and low gain modes only. This form of gain control is not smooth, and the gain cannot be continuously tuned to optimally track received signal strength. Some conventional LNAs adjust the gain by tuning the bias current. However, when the bias voltage is changed, the LNA's linearity changes, and there is no control for this. Other conventional LNAs employ an attenuation network to adjust the gain, which introduces additional noise. Still other conventional LNAs utilize a bypass topology to adjust the gain. The gain step is too large, and cannot be tuned smoothly.
Most conventional LNAs use a fixed transistor size. Once the topology is chosen and the size of the transistor is determined, it cannot be dynamically altered. This precludes many optimizations, such as smooth gain tuning, low noise and high linearity, and low power consumption.